Method for producing flexo printing forms by means of laser direct engraving

ABSTRACT

Flexographic printing plates are produced by means of direct laser engraving by a process in which the starting material used is a flexographic printing element, the relief-forming layer of which has a combination of a substantially hydrophobic, elastomeric binder and an inert plasticizer. Flexographic printing plates obtainable by this process are used for flexographic printing with water-based or alcohol-based printing inks.

This application is the US national phase of international applicationPCT/EP03/06331 filed 16 Jun. 2003 which designated the U.S. and claimsbenefit of DE 102 27 189.5, dated 18 Jun. 2002, the entire content ofwhich is hereby incorporated by reference.

The present invention relates to a process for the production offlexographic printing plates by means of direct laser engraving, inwhich the starting material used is a flexographic printing element, therelief-forming layer of which has a combination of a substantiallyhydrophobic, elastomeric binder and an inert plasticizer. The presentinvention furthermore relates to flexographic printing plates obtainableby this process and the use of flexographic printing plates forflexographic printing with water-based or alcohol-based printing inks.

Lasers are now used both in the area of offset printing plates and inthe area of relief printing plates for various steps of the productionprocess.

For example, it is known that the photosensitive layers of offsetprinting plates can be inscribed imagewise by means of suitable laserexposure units. The photosensitive layer is chemically modified, forexample crosslinked, by the laser. The finished offset printing plate isobtained from the image-bearing crude product by means of a suitabledevelopment process (cf. for example Imaging Technology, Section3.4.1.2., Ullmann's Encyclopedia of Industrial Chemistry, 6^(th) Edt.,2000 Electronic release). The thickness of said photosensitive layers ofoffset printing plates is usually from 0.3 to 5 μm.

It is furthermore known that images can be produced from flexographicprinting plates with the use of IR-ablative masks, as disclosed, forexample, in EP-A 654 150, instead of photographically produced masks.Here, a thin IR-sensitive, opaque layer is applied to thephotopolymerizable layer. The thickness of such IR-ablative layers isusually just a few μm. The IR-ablative layer is inscribed imagewiseusing an IR laser, i.e. the parts in which the laser beam is incident onit are removed. The actual printing relief-forming is produced in theconventional manner: exposure is effected to actinic light through themask produced, and the relief layer is thus selectively crosslinked.Development is then effected with a developer in a conventional manner,both photosensitive material from the unexposed parts of therelief-forming layer and the residues of the IR-ablative layer beingremoved. Since the IR-ablative mask layer is of no importance for theactual printing process, the materials therefor can be soughtexclusively with regard to the optimum use as a mask.

In direct laser engraving for the production of flexographic printingplates, on the other hand, a printing relief is engraved directly intothe relief-forming layer of a flexographic printing element by means ofa laser. A subsequent development step, as in the case of conventionalplates or in the mask process, is no longer required. Typical relieflayer thicknesses of flexographic printing plates are from 0.5 to 7 mmand may also be 0.2 mm in the case of special thin-film plates. Thenonprinting wells in the relief are at least 0.03 mm in the screen areaand substantially more in the case of other negative elements and mayassume values up to 3 mm in the case of thick plates. Thus, largeamounts of material have to be removed by means of the laser.

EP-A 640 043 and EP-A 640 044 disclose one-layer or multilayerelastomeric laser-engravable flexographic printing elements for theproduction of flexographic printing plates by means of laser engraving.The elements consist of reinforced elastomeric layers. Elastomericbinders are used for the production of the layer. The mechanicalstrength of the layer is increased by the reinforcement, in order topermit flexographic printing. The reinforcement is achieved either byintroduction of suitable fillers, photochemical or thermochemicalcrosslinking or combinations thereof.

U.S. Pat. No. 5,259,311 discloses a process in which a commercialflexographic printing element is photochemically crosslinked by uniformexposure to UV/A in a first step, the release layer is then removedusing a flexographic washout agent and a printing relief is engraved bymeans of a laser in a second step. A cleaning step is then carried outby means of a flexographic washout agent, followed by final drying ofthe plate.

Although the engraving of rubber impression cylinders by means of lasershas in principle been known since the 60s of the last century and thepatents cited have also been filed 10 years ago, laser engraving hasacquired broader commercial interest only in recent years with theadvent of improved laser systems. The improvements in the laser systemsinclude better focusability of the laser beam, higher power andcomputer-controlled beam modulation.

With the introduction of new, more efficient laser systems, however, thequestion of particularly suitable materials for laser-engravableflexographic printing plates is becoming increasingly important.Problems which played no role at all in the past because the lasersystems did not at all allow the engraving of very fine structures arenow important and lead to new requirements with respect to the material.

The relief layers of flexographic printing plates are of course soft andhave relatively low melting or softening points. In laser engraving,they therefore have a strong tendency to form melt edges around theengraved elements. At the edge of the engraved elements, the layer meltsunder the influence of the laser beam but is not, or not completely,decomposed. Such melt edges cannot be removed or at least cannot becompletely removed even by subsequent washing and lead to a blurredprint. Undesired melting of the layer furthermore results in reducedresolution of the print motif in comparison with the digital datarecord.

EP-A 1 136 254 proposes the use of polyoxyalkylene/polyethylene glycolgraft copolymers as binders for relief-forming layers for solving thisproblem. However, since these copolymers are water-soluble, such reliefprinting plates have the disadvantage that they can be used only to alimited extent. The relief layer swells to an excessive extent inwater-based flexographic printing inks, so that undesired effects, forexample an intolerable increase in tonal value, occur during printing.Such printing plates can therefore be used substantially only forprinting with UV inks. There is an urgent need to providelaser-engravable relief printing elements which are also suitable forprinting with water-based inks and nevertheless can be engraved withlasers without undesirably strong melting of the layer.

Furthermore, the degradation products which form in the course of thelaser engraving frequently give rise to problems. In addition to gaseousfractions, aerosols are also produced. These are as a rule extremelytacky and may be wholly or partly deposited again on the surface of theprinting relief and, in unfavorable cases, can even react again with thesurface. This leads to unclean surfaces and hence also to poor printingbehavior.

For solving this problem, U.S. Pat. No. 5,259,311 proposes subsequentlycleaning the surface of the relief printing plate after the laserengraving with the aid of an organic solvent. However, the tackydecomposition products have substantially the same solubility behavioras the relief layer. For relief layers comprising hydrophobic polymers,an organic solvent therefore also has to be used for removing thedecomposition products. The crosslinked relief layer is no longersoluble therein but may well still be swellable. After such a subsequentwashing step, the layer therefore has to be dried again in a furtherprocess step. The time and handling advantage achieved by laserengraving in the process is eliminated again since the drying processtakes the most time in the course of processing. Decomposition productswhich have reacted again with the surface can no longer be removed atall and are consequently also detectable in the print. It will beextremely desirable to be able to have a flexographic printing elementin which possible deposits can be removed simply with water or aqueouscleaning agents without the plate swelling thereby.

Very rapid engraving is furthermore required for the economicalproduction of flexographic printing plates by means of laser engraving.The speed of the engraving depends on the one hand on the laser systemchosen. On the other hand, the sensitivity of the relief-forming layerto the laser radiation chosen in each case should be very high. Withregard to the sensitivity, however, it should be taken into account thatthe relief layer of the flexographic printing plate imparts both theelastomeric properties and the typical printing properties. Measures forimproving the sensitivity therefore must not impair said properties.

It is an object of the present invention to provide a process for theproduction of flexographic printing plates by means of direct laserengraving, in which the occurrence of melt edges is substantiallyreduced, possible deposits of decomposition products can be removed bysimple treatment of the plate with water or aqueous cleaning agents andvery rapid engraving with high resolution is made possible and in whichthe flexographic printing plates obtained are moreover suitable forprinting with water-based flexographic printing inks.

We have found that this object is achieved by a process for theproduction of flexographic printing plates by means of laser engraving,in which the starting material used is a crosslinkable, laser-engravableflexographic printing element which at least comprises, arranged one ontop of the other,

-   -   a dimensionally stable substrate,    -   at least one crosslinkable, laser-engravable relief-forming        layer having a thickness of at least 0.2 mm, at least comprising        a substantially hydrophobic, elastomeric binder, a plasticizer        and crosslinkable components        which process comprises at least the following steps:

-   (a) uniform crosslinking of the relief-forming layer and

-   (b) engraving of a printing relief into the crosslinked    relief-forming layer with the aid of a laser, the height of the    relief elements engraved with the laser being at least 0.03 mm,    -   the binder plasticizer being an inert plasticizer.

Flexographic printing plates which are obtainable by the processdescribed and the use of these flexographic printing plates forflexographic printing with water-based and/or alcohol-based printinginks have furthermore been found.

Surprisingly, it has been found that flexographic printing elementswhich have excellent sensitivity to lasers are obtained by the novelcombination of a substantially hydrophobic, elastomeric binder withinert plasticizers. The relief-forming layer scarcely melts under theinfluence of the laser radiation, and scarcely any melt edges formaround the negative elements.

Regarding the present invention, the following may be statedspecifically:

Examples of suitable dimensionally stable substrates for theflexographic printing elements used as starting materials for theprocess are plates, sheets and conical and cylindrical sleeves ofmetals, such as steel, aluminum, copper or nickel, or of plastics, suchas polyethylene terephthalate (PET), polyethylene naphthalate (PEN),polybutylene terephthalate, polyamide, polycarbonate, if required alsowoven fabrics and nonwovens, such as glass fiber fabrics, and compositematerials, for example of glass fibers and plastics. Particularlysuitable dimensionally stable substrates are dimensionally stablesubstrate sheets, for example polyester sheets, in particular PET or PENsheets, or flexible metallic substrates, such as thin metal sheets ormetal foils of steel, preferably of stainless steel, magnetizable springsteel, aluminum, zinc, magnesium, nickel, chromium or copper.

The flexographic printing element furthermore comprises at least onelaser-engravable, crosslinkable relief-forming layer. The crosslinkablerelief-forming layer may be applied directly on the substrate. However,other layers, for example adhesion-promoting layers and/or resilientlower layers, may also be present between the substrate and therelief-forming layer.

The crosslinkable relief-forming layer comprises at least onesubstantially hydrophobic, elastomeric binder, crosslinkable componentsand at least one inert plasticizer. As a rule, the crosslinkablerelief-forming layer as a whole already has elastomeric properties; forthe present invention, however, it is sufficient if the crosslinkedlayer first has the elastomeric properties typical of a flexographicprinting plate.

The substantially hydrophobic elastomers are those which are usuallyused for the preparation of conventional flexographic printing platesdevelopable in an organic medium and which are neither soluble norswellable in water. Examples are natural rubber, polybutadiene,polyisoprene, styrene/butadiene rubber, nitrile/butadiene rubber, butylrubber, styrene/isoprene rubber, polynorbornene rubber orethylene/propylene/diene rubber (EPDM).

The substantially hydrophobic elastomer is preferably a thermoplasticelastomeric block copolymer of alkenylaromatics and 1,3-dienes. Theblock copolymers may be both linear block copolymers and radial blockcopolymers. They are usually three-block copolymers of the A-B-A typeand may also be two-block polymers of the A-B type, or those having aplurality of alternating elastomeric and thermoplastic blocks, e.g.A-B-A-B-A. Mixtures of two or more different block copolymers may alsobe used. Commercial three-block copolymers frequently contain certainamounts of two-block copolymers. The diene units may be 1,2- or1,4-linked. Both block copolymers of the styrene/butadiene type and ofthe styrene/isoprene type may be used. They are commercially available,for example, under the name Kraton®. Thermoplastic elastomeric blockcopolymers having terminal blocks comprising styrene and a randomstyrene/butadiene middle block, which are available under the nameStyroflex®, may furthermore be used. The block copolymers may also becompletely or partly hydrogenated, as, for example, in SEBS rubbers.

Of course, mixtures of a plurality of binders may also be used, providedthat the properties of the relief-forming layer are not adverselyaffected thereby. The total amount of binders is usually from 40 to 80,preferably from 40 to 70, particularly preferably from 40 to 65, % byweight, based on the sum of all components of the relief layer.

For the novel process, the substantially hydrophobic binder is used as amixture with at least one inert plasticizer.

In the context of this invention, inert means that the plasticizers haveno or at least substantially no polymerizable groups which can react inthe course of free radical crosslinking of the relief layer in such away that the plasticizers are also incorporated into the polymericnetwork of the relief-forming layer. Inert plasticizers have inparticular substantially no ethylenically unsaturated double bonds.

It is of course known to a person skilled in the art that in principlealso single C—H bonds can react by the chain transfer route in thecourse of free radical polymerization. However, this is not intended tocontradict the term inert, since it is also known to a person skilled inthe art that this reaction will take place only to a minor extentcompared with the reaction of ethylenically unsaturated double bonds.

Examples of suitable inert plasticizers include in particular alkylesters of alkanecarboxylic acids, in particular alkanedicarboxylicacids, arylcarboxylic acids or phosphoric acid. Preferred alcoholiccomponents of the esters are straight-chain or branched C₈- toC₂₀-alkanols, particularly preferably C₈- to C₁₃-alkanols, such asn-octanol, 2-ethylhananol, n-nonanol, isononanol, n-decanol, isodecanol,n-undecanol, isoundecanol, n-dodecanol, isododecanol, n-tridecanol andisotridecanol. The term “iso” alkanols is understood in the case of saidcompounds as meaning a mixture of different isomers which are usuallyobtained in the industrial synthesis of the alkanols. Preferredcarboxylic components in the esters are in particular alkanedicarboxylicacids of at least 6 carbon atoms, for example adipic acid, azelaic acid,sebacic acid and phthalic acid. Suitable diesters may be bothsymmetrical esters and those which have two different alcoholic groups.Examples of ester-based inert plasticizers include di-2-ethylhexylphthalate, di-2-ethylhexyl adipate, diisononyl adipate, diisodecylphthalate, diisoundecyl phthalate, undecyl dodecyl phthalate, ditridecylphthalate and ditridecyl adipate.

Further examples of inert plasticizers include high-boiling paraffinic,naphthenic and aromatic mineral oils. Such mineral oils are obtained bydistillation of mineral oils under reduced pressure.

High-boiling substantially paraffinic and/or naphthenic mineral oils arepreferred. Such mineral oils are also referred to as white oils, aperson skilled in the art distinguishing between technical-grade whiteoils which can still have a low content of aromatics, and medical whiteoils, which are substantially free of aromatics. They are commerciallyavailable, for example Shell Risella (technical-grade white oil) orShell Ondina (medical white oil).

Medical white oils are very particularly preferred.

Of course, mixtures of different plasticizers may also be used, providedthat the properties of the relief-forming layer are not adverselyaffected thereby.

The amount of inert plasticizer is used by a person skilled in the artin effective amounts depending on the desired properties of the layer.As a rule, at least 5% by weight, based on the sum of all components ofthe relief layer, of inert plasticizer are required. This does not ofcourse exclude the possibility that, in exceptional cases, effectiveresults can also be achieved in storage engraving with smaller amounts.As a rule, the amount of inert plasticizer is from 5 to 40, preferablyfrom 10 to 40, particularly preferably from 20 to 40, % by weight, basedon the sum of all components of the layer.

The type and amount of the components for the crosslinking of the layerdepend on the desired crosslinking technique and are chosen accordinglyby a person skilled in the art. The uniform crosslinking of thecrosslinkable relief layer is, in particular, carried outphotochemically or thermochemically. The crosslinking is preferablycarried out photochemically.

In the case of the photochemical crosslinking, the relief-forming layercomprises at least one photoinitiator or a photoinitiator system andsuitable monomers or oligomers.

Benzoin and benzoin derivatives, such as α-methylbenzoin and benzoinethers, benzil derivatives, such as benzil ketals, acylarylphosphineoxides, acylarylphosphinic esters and polynuclear quinones are suitablein a known manner as initiators for the photopolymerization, there beingno intention to restrict the list to these.

The monomers have at least one polymerizable, olefinically unsaturatedgroup. Esters or amides of acrylic acid or methacrylic acid with mono-or polyfunctional alcohols, amines, aminoalcohols or hydroxyethers andhydroxyesters, styrene or substituted styrenes, esters of fumaric ormaleic acid or allyl compounds have proven particularly advantageous.Examples of suitable monomers include butyl acrylate, 2-ethylhexylacrylate, lauryl acrylate, 1,4-butanediol diacrylate, 1,6-hexanedioldiacrylate, 1,6-hexanediol dimethacrylate, 1,9-nonanediol diacrylate,trimethylolpropane triacrylate, dioctyl fumarate and N-dodecylmaleimide.Suitable oligomers having olefinic groups may also be used. It is ofcourse also possible to use mixtures of different monomers or oligomers,provided that no undesired effects occur. The total amount of themonomers is established by a person skilled in the art according to thedesired properties of the layer. As a rule, however, 20% by weight,based on the amount of all components of the laser-engravablerelief-forming layer, should not be exceeded.

Thermal crosslinking can, on the one hand, be carried out analogously tothe photochemical crosslinking, by using a thermal polymerizationinitiator instead of a photoinitiator. Commercial thermal initiators forfree radical polymerization, for example peroxides, hydroperoxides orazo compounds, are in principle suitable. The thermal crosslinking mayalso be carried out by adding a heat-curable resin, for example an epoxyresin, as a crosslinking component to the layer.

The crosslinkable relief-forming layer can optionally furthermorecomprise an absorber for laser radiation. Mixtures of differentabsorbers for laser irradiation may also be used. Suitable absorbers forlaser radiation have a high absorption in the region of the laserwavelength. Particularly suitable absorbers are those which have a highabsorption in the near infrared and in the longer-wave VIS range of theelectromagnetic spectrum. Such absorbers are particularly suitable forthe absorption of the radiation of Nd-YAG lasers (1 064 nm) and of IRdiode lasers, which typically have wavelengths of from 700 to 900 nm andfrom 1 200 to 1 600 nm.

Examples of suitable absorbers for laser radiation are dyes which absorbstrongly in the infrared spectral range, for example phthalocyanines,naphthalocyanines, cyanines, quinones, metal complex dyes, such asdithiolenes, or photochromic dyes. Further suitable absorbers areinorganic pigments, in particular intensely colored inorganic pigments,for example chromium oxides, iron oxides, carbon black or metallicparticles. Particularly suitable absorbers for laser radiation arefinely divided carbon black grades having a primary particle size offrom 10 to 50 nm.

The amount of the optionally added absorber is chosen by a personskilled in the art according to the respective desired properties of thelaser-engravable flexographic printing element. In this context, aperson skilled in the art will take into account the fact that the addedabsorbers influence not only the engraving of the elastomeric layer bylaser but also other properties of the relief printing plate obtained asthe end product of the process, for example its hardness, resilience,thermal conductivity or ink transfer behavior. As a rule, it istherefore advisable to use not more than 20% by weight at most,preferably not more than 10% by weight, based on the sum of allcomponents of the layer, of absorber for laser radiation.

As a rule, it is not advisable to add to relief-forming layers which areto be photochemically crosslinked absorbers for laser radiation whichalso absorb in the UV range, since the photopolymerization is at leastgreatly impaired thereby and may be rendered completely impossible. Itis advisable as a rule to subject such relief layers containing laserabsorbers to thermal crosslinking.

The relief-forming layer may furthermore comprise additives andassistants, for example dyes, dispersants or antistatic agents. However,the amount of such additives should as a rule not exceed 5% by weight,based on the amount of all components of the crosslinkable,laser-engravable layer of the recording element.

The crosslinkable relief-forming layer may also be composed of aplurality of part-layers. These crosslinkable part-layers may be of thesame, roughly the same or different material composition.

The thickness of the laser-engravable, elastomeric relief-forming layeris at least 0.2, preferably from 0.3 to 7, particularly preferably from0.5 to 5, very particularly preferably from 0.7 to 4, mm. The thicknessis suitably chosen by a person skilled in the art according to thedesired use of the flexographic printing plate.

In a preferred embodiment, the starting material comprises an additionallaser-engravable polymer layer which is soluble or at least swellable inaqueous media and is arranged on the laser-engravable relief layer, andwhich comprises at least one polymer soluble or swellable in aqueoussolvents. Such a layer serves for facilitating a subsequent cleaningstep optionally to be carried out. Solid decomposition products formedin the course of the laser engraving may be deposited on this auxiliarylayer and can be more easily removed.

Examples of the polymer soluble or at least swellable in aqueoussolvents include polyvinyl alcohol, polyvinyl alcohol/polyethyleneglycol graft copolymers, polyvinylpyrrolidone and its derivatives andcellulose derivatives, in particular cellulose esters and celluloseethers, such as methylcellulose, ethylcellulose, benzylcellulose,hydroxyalkylcelluloses or nitrocelluloses. Mixtures of a plurality ofpolymers can of course also be used.

The additional laser-engravable polymer layer may also contain additivesand assistants, for example plasticizers or laser absorbers. If it isintended to crosslink the laser-engravable relief layer photochemically,the additional polymer layer should as far as possible be transparent inthe UV range. In the case of other crosslinking methods, this is notabsolutely essential.

The thickness of the additional polymer layer should be very small. Itdepends substantially on the depth of focus of the laser used forengraving in the process. It is limited so that there is no substantialbroadening of the focus on the surface of the relief layer. Thethickness of such an additional polymer layer should as a rule notexceed 100 μm. As a rule, satisfactory results are no longer achieved inthe case of greater thicknesses. The thickness should preferably notexceed 50 μM. The thickness is particularly preferably 1-40 μm, veryparticularly preferably 2-25 μm.

The laser-engravable flexographic printing element can optionally alsocomprise further layers.

Examples of such layers include an elastomeric lower layer comprising adifferent formulation, which is present between the substrate and thelaser-engravable layer or layers and which need not necessarily belaser-engravable. The mechanical properties of the relief printingplates can be modified by means of such lower layers without theproperties of the actual printing relief layer being influenced.

Resilient substructures which are present under the dimensionally stablesubstrate of the laser-engravable flexographic printing element, i.e. onthat side of the substrate which faces away from the laser-engravablelayer, serve the same purpose.

Further examples include adhesion-promoting layers which bond thesubstrate to layers located above or bond different layers to oneanother.

Furthermore, the laser-engravable flexographic printing element can beprotected from mechanical damage by a protective sheet—also called coversheet—which consists, for example, of PET and is present on therespective uppermost layer and which has to be removed before engravingby means of lasers. To facilitate peeling off, the protective sheet mayhave been surface-treated in a suitable manner, for example bysiliconizing, provided that the top relief layer is not adverselyaffected in its printing properties by the surface treatment.

The flexographic printing element used as a starting material for theprocess can be produced, for example, by dissolving or dispersing allcomponents in a suitable solvent and casting on a substrate. In the caseof multilayer elements, a plurality of layers can be cast one on top ofthe other in a manner known in principle. After the casting, the coversheet can, if desired, be applied for protecting the starting materialfrom damage. Conversely, it is also possible to cast onto the coversheet and finally to laminate with the substrate. The casting method isparticularly advisable if thermal crosslinking is intended.

If thermoplastic elastomeric binders are used, the production of theflexographic printing element can particularly advantageously in amanner known in principle by melt extrusion between a substrate sheetand a cover sheet or a cover element and calendering of the compositeobtained, as disclosed, for example, in EP-A 084 851. This method isparticularly advisable if crosslinking is to be effected photochemicallyor by means of electron beams. In this way, it is also possible toproduce thick layers in a single operation. Multilayer elements can beproduced by means of coextrusion. Flexographic printing elements havingmetallic substrates can preferably be obtained by casting or extrudingonto a temporary substrate and then laminating the layer with themetallic substrate.

The application of the additional polymer layer can be effected, forexample, by dissolving the components in a suitable solvent and castingonto the relief-forming layer. Preferably, however, the cover sheet iscoated with the additional polymer layer and laminated with the relieflayer or used as a sheet for the extrusion process.

In the novel process, the starting material is first uniformlycrosslinked in the first process step (a).

The uniform crosslinking of the crosslinkable relief layer can becarried out photochemically, in particular by exposure to UV-A radiationhaving a wavelength of from 320 to 400 nm or UV-A/VIS radiation having awavelength of from about 320 to about 700 nm. Uniform thermochemicalcrosslinking is effected by very uniform heating of the relief-forminglayer at constant temperature.

The photochemical crosslinking is particularly suitable for layers whichcontain no strongly colored absorbers for laser radiation and aretransparent or at least substantially transparent in the UV/VIS range.However, transparent layers can of course also be crosslinkedthermochemically. Layers containing colored laser absorbers canadvantageously be crosslinked thermochemically.

The uniform crosslinking may also be carried out by means of electronbeams.

Of course, the flexographic printing element used as a starting materialfor the process is usually produced by a printing plate manufacturerwhereas the laser engraving is carried out by process engravers orprinting works. The uniform crosslinking (a) can on the one hand becarried out by the process engravers themselves. For example, thephotochemical crosslinking can be carried out in commercial exposureunits for flexographic printing plates. On the other hand, thecrosslinking can of course also be effected by the manufacturer offlexographic printing elements or on his premises.

In process step (b), a printing relief is engraved into the crosslinkedrelief-forming layer by means of a laser. If a protective sheet ispresent, this is removed prior to engraving.

The term laser-engravable is to be understood as meaning that the relieflayer has the property of absorbing laser radiation, in particular theradiation of an IR laser, so that it is removed or at least detached inthose parts where it is exposed to a laser beam of sufficient intensity.The layer is preferably vaporized or thermally or oxidatively decomposedwithout melting beforehand, so that its decomposition products areremoved from the layer in the form of hot gases, vapors, fumes or smallparticles.

IR lasers are particularly suitable for engraving. For example, a CO₂laser having a wavelength of 10.6 μm may be used. Furthermore, Nd-YAGlasers (1 064 nm), IR diode lasers or solid-state lasers may be used. Itis also possible to use lasers having shorter wavelengths, provided thatthe laser has a sufficient intensity. For example, a frequency-doubled(532 nm) or frequency-tripled (355 nm) Nd-YAG laser or an excimer laser(e.g. 248 nm) may also be used.

The addition of absorbers for laser radiation depends substantially onthe type of laser which is to be used for the engraving. As a rule, thesubstantially hydrophobic, elastomeric binders used for therelief-forming layer absorb the radiation of CO₂ lasers to a sufficientextent, so that additional IR absorbers in the relief layer are as arule not required when this type of laser is used. The same applies toUV lasers, for example excimer lasers. In the case of Nd-YAG lasers andIR diode lasers, the addition of a laser absorber is generallynecessary.

The image information to be engraved can be transferred directly fromthe layout computer system to the laser apparatus. The lasers can beoperated either continuously or in pulsed mode.

Relief elements in which the sidewalls of the elements initially dropperpendicularly and broaden only in the lower region are advantageouslyengraved. A good shoulder shape of the relief dots together with littleincrease in tonal value is thus achieved. However, sidewalls of otherdesigns can also be engraved.

The height of the elements to be engraved depends on the total thicknessof the relief and on the type of elements to be engraved and isdetermined by a person skilled in the art according to the desiredproperties of the printing plate. The height of the relief elements tobe engraved is at least 0.03 mm, preferably at least 0.05 mm, theminimum depth between individual dots being mentioned here. Printingplates having relief heights which are too small are as a ruleunsuitable for printing by means of a flexographic printing technique,because the negative elements become full to overflowing with printingink. Individual negative dots should usually have greater depths; forthose of 0.2 mm diameter, a depth of at least from 0.07 to 0.08 mm isusually advisable. In the case of surfaces which have been removed byengraving, a depth of more than 0.15 mm, preferably more than 0.4 mm, isadvisable. The latter is of course possible only in the case of anappropriately thick relief.

Advantageously, the flexographic printing plate obtained is cleaned in afurther process step (c) after the laser engraving. In some cases, thiscan be effected by simply blowing off with compressed air or brushingoff.

However, a liquid cleaning agent is preferably used for the subsequentcleaning, in order also to be able to remove polymer fragmentscompletely. This is particularly advisable, for example, when foodpackaging which has to meet particularly stringent requirements withrespect to volatile components is to be printed using the flexographicprinting plate.

The subsequent cleaning can be very particularly advantageously effectedby means of water or an aqueous cleaning agent. Aqueous cleaning agentssubstantially comprise water and optionally small amounts of alcoholsand may contain assistants, for example surfactants, emulsifiers,dispersants or bases, for promoting the cleaning process. It is alsopossible to use mixtures which are usually used for developingconventional, water-developable flexographic printing plates. Since therelief layer comprising the substantially hydrophobic, elastomericbinder is not swellable in water, time-consuming drying of the printingplate is avoided by the use of water or aqueous cleaning agents.

The subsequent cleaning can be effected, for example, by simpleimmersion or spraying of the relief printing plate or can additionallybe promoted by mechanical means, for example by brushing or treatmentwith a plush pad. It is also possible to use conventional flexographicplate washers.

In the subsequent washing step, any deposits and the residues of theadditional polymer layer are removed. This layer advantageously preventspolymer droplets formed in the course of the laser engraving frombecoming firmly bonded again to the surface of the relief layer, or atleast makes it more difficult for this to occur. Deposits can thereforebe particularly readily removed. It is as a rule advisable to carry outthe subsequent washing step immediately after the laser engraving step.

Although not the preferred variant, it is also possible in principle touse mixtures of organic solvents for the subsequent cleaning, inparticular those mixtures which usually serve as washout agents forconventionally produced flexographic printing plates. Examples includewashout agents based on high-boiling, dearomatized mineral oilfractions, as disclosed, for example, in EP-A 332 070, or water-in-oilemulsions, as disclosed in EP-A 463 016. This variant can be used inparticular when no additional polymer layer is present. If an additionalpolymer layer is present but cannot be removed with the organic solventused, cleaning must additionally be effected with water or an aqueouscleaning agent.

The flexographic printing plates obtained are particularly suitable forprinting with water-based inks and alcohol-based inks. However, they areof course also suitable for printing with UV inks or flexographicprinting inks which contain small amounts of esters.

The examples which follow illustrate the invention:

General Preparation Method for the Starting Material:

A photochemically crosslinkable laser-engravable relief-forming layerwas produced from, in each case, 55% by weight (based on the sum of allcomponents) of a hydrophobic elastomeric binder (Kraton D-1102, SBSblock copolymer), 32% by weight of a plasticizer, 10% by weight ofhexanediol diacrylate, 2% by weight of photoinitiator and 1% of dye andheat stabilizer.

The components were processed using an extruder (ZSK 53) at 140° C.,introduced by means of a slot die between a dimensionally stable PETsubstrate sheet and a PET protective sheet and then calendered by meansof a two-roll calender. The thickness of the resulting crosslinkable,laser-engravable layer was in each case 1.14 mm.

Plasticizers used were the plasticizers shown in table 1. Inertplasticizers of substantially paraffinic mineral oils, which have noethylenically unsaturated double bonds, were used for examples 1 and 2,and polybutadiene oils which have ethylenically unsaturated double bondsin the chain or in the side groups were used for the comparativeexamples.

TABLE 1 Plasticizers used for experiments and comparative experimentsType Example 1 White oil S 5000 (BASF) Example 2 Medical white oil(Ondina oil G 34, from Shell) Comparative Polybutadiene oil (Nisso PB1000, from Nippon example 1 Soda Co., Ltd.) Comparative Polybutadieneoil (Polyöl 130, from Degussa) example 2Carrying Out the Novel Process:

The protective PET sheet was peeled off from the laser-engravableflexographic printing elements obtained in the examples and comparativeexamples. They were uniformly crosslinked by exposure to UVA light for20 minutes in a first process step. In examples 1 and 2, additionalcrosslinking of the uppermost region of the relief layer was carried outusing UVC light.

Laser Engraving of the Flexographic Printing Elements

A CO₂ laser (from ALE, type “ALE meridian finesse”) having a spotdiameter of about 30 μm and a rated power of 250 watt was used for laserengraving experiments. The power on the plate surface at maximum powerwas 150 watt. The laser engraving experiments were carried out using thefollowing software parameters: Total relief=75, First step=48, Engravingspeed=240 rpm and Shoulder base width=1.24.

After the flexographic printing element had been clamped on a cylinder,a test motif consisting of various, typical, positive and negativeelements was engraved into the flexographic printing element. Inaddition to surface areas completely removed by engraving and 100% tonalvalues, the motif also contained various screen areas having tonalvalues of from 1 to 98% and 40 μm wide negative lines in the axial andtransverse directions relative to the axis of rotation of the cylinder.

Results

The engraving depth was from 0.64 to 0.685 mm in the case of allflexographic printing plates. However, the plates of examples 1 and 2which were produced according to the invention and comprised inertplasticizers had substantially no melt edges, whereas the plates ofcomparative examples 1 and 2 comprising reactive plasticizers hadsubstantial melt edges in comparison therewith.

After the laser engraving, the flexographic printing plates obtainedwere washed for two minutes with a mixture of water and a surfactantwith simultaneous brushing of the surface. A nyloprint® washer(apparatus combination CW 22×30, BASF Drucksysteme GmbH) was used forthis purpose.

The plates comprising inert plasticizer were washed for 5 minutes andthose comprising reactive plasticizer for 10 minutes at 60° C.Nevertheless, in spite of twice the washing time a substantial residueof removed material is still detectable on the flexographic printingplates comprising reactive plasticizers, whereas the flexographicprinting plates comprising the inert plasticizers used according to theinvention have been cleaned so that no residue is left.

The flexographic printing plates obtained are suitable for printing withalcohol-based and water-based inks.

1. A process for the production of flexographic printing plates by meansof laser engraving, in which the starting material used is acrosslinkable, laser-engravable flexographic printing element which atleast comprises, arranged one on top of the other, a dimensionallystable substrate, at least one crosslinkable, laser-engravablerelief-forming layer having a thickness of at least 0.2 mm, at leastcomprising an essentially hydrophobic elastomeric binder, a plasticizerand components for crosslinking, and which process comprises at leastthe following steps: (a) uniform crosslinking of the relief-forminglayer and (b) engraving of a print relief into the crosslinked relieflayer with the aid of a laser, the height of the relief elements to beengraved with the laser being at least 0.03 mm, wherein the plasticizeris an inert plasticizer selected from the group consisting of aromatic,naphthenic and paraffinic mineral oils.
 2. A process as claimed in claim1, wherein the inert plasticizer is a paraffinic and/or naphthenicmineral oil.
 3. A process as claimed in claim 1, wherein the plasticizeris used in an amount of from 5 to 40% by weight, based on the amount ofall components of the relief-forming layer.
 4. A process as claimed inclaim 3, wherein the plasticizer is used in an amount of from 20 to 40%by weight, based on the amount of all components of the relief-forminglayer.
 5. A process as claimed in claim 1, wherein the binder is athermoplastic elastomeric binder.
 6. A process as claimed in any claim1, wherein the uniform crosslinking (a) is carried out photochemicallyor thermally.
 7. A process as claimed in claim 1, wherein therelief-forming layer additionally comprises an absorber for laserradiation.
 8. A process as claimed in claim 1, wherein the flexographicprinting element comprises an additional, water-soluble laser-engravablelayer which is arranged on the laser-engravable relief-forming layer andcomprises at least one polymer soluble or swellable in aqueous solventsand which is removed after process step (b) in a further process step(c) by means of water or an aqueous cleaning agent.
 9. A process asclaimed in claim 8, wherein the polymer is at least one polymer selectedfrom the group consisting of polyvinyl alcohol, polyvinylalcohol/polyethylene glycol graft copolymers, polyvinylpyrrolidone andcellulose derivatives.
 10. A flexographic printing plate obtainable by aprocess as claimed in claim
 1. 11. The use of a flexographic printingplate as claimed in claim 10 for flexographic printing with water-basedand/or alcohol-based printing inks.